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Ch. 3: Atoms
3.1 Foundations
History

Democritus
 named the most basic
particle
 atom- means “indivisible”

Aristotle
 didn’t believe in atoms
 thought matter was
continuous (thought you
could keep dividing it)
History
 by


1700s, all chemists agreed:
on the existence of atoms
that atoms combined to make
compounds
 Still
did not agree on whether
elements combined in the same ratio
when making a compound
Law of Conservation of Mass
 mass
is neither created or destroyed
during regular chemical or physical
changes
Law of Definite Proportions
 any
amount of a compound contains
the same element in the same
proportions by mass
No matter
where the
copper
carbonate is
used, it still
has the same
composition
Law of Multiple Proportions
applies when 2 or more elements combine
to make more than one type of compound
 the mass ratios of the second element
simplify to small whole numbers

15.9994
15.9994
15.9994
15.9994
31.9988
Law of Multiple Proportions
Dalton’s Atomic Theory
1.
2.
3.
4.
5.
All matter is made of atoms
Atoms of same element have the same
size, mass, and properties
Atoms can’t be subdivided, created or
destroyed
Atoms of different elements combine in
whole number ratios to make compounds
In chemical reactions, atoms can be
combined, separated, and rearranged.
Modern Atomic Theory

Some parts of Dalton’s theory were wrong:



atoms are divisible into smaller particles
(subatomic particles)
atoms of the same element can have different
masses (isotopes- atoms of the same type with
different masses)
Most important parts of atomic theory:


all matter is made of atoms
atoms of different elements have different
properties
Ch. 3: Atoms
3.2 Structure of Atom
STRUCTURE OF THE ATOM
 ALTHOUGH
DALTON THOUGHT ATOMS
WERE INDIVISIBLE, INVESTIGATORS IN
THE LATE 1800’S PROVED OTHERWISE
 IT SOON BECAME CLEAR THAT ATOMS
ARE ACTUALLY COMPOSED OF SEVERAL
BASIC TYPES OF SMALLER PARTICLES
 AND IT’S THE NUMBERS AND
ARRANGEMENTS OF THESE SUBATOMIC PARTICLES THAT DETERMINES
THE IDENTITY OF THE ATOM.
STRUCTURE OF THE ATOM
 THE
FIRST DISCOVERY OF A
SUBATOMIC PARTICLE RESULTED FROM
THE INVESTIGATIONS INTO THE
RELATIONSHIP BETWEEN ELECTRICITY
AND MATTER.
 IN THE LATE 1800’S, MANY
EXPERIMENTS WERE PERFORMED IN
WHICH ELECTRIC CURRENT WAS
PASSED THROUGH VARIOUS GASES AT
LOW PRESS

CARRIED OUT IN
TUBES CALLED
CATHODE-RAY TUBES
STRUCTURE OF THE ATOM
 INVESTIGATORS
NOTICED THAT WHEN
CURRENT WAS PASSED THROUGH A
CATHODE RAY TUBE, THE SURFACE OF
THE TUBE DIRECTLY OPPOSITE THE
CATHODE GLOWED.
STRUCTURE OF THE ATOM
 THEY
HYPOTHESIZED THAT THE GLOW
WAS CAUSED BY A STREAM OF
PARTICLES

THEY CALLED THE STREAM A CATHODE
RAY
STRUCTURE OF THE ATOM
 THE
CATHODE RAY TRAVELED FROM
THE CATHODE TO THE ANODE WHEN
CURRENT WAS PASSED THROUGH THE
TUBE.
 THE CATHODE RAY TUBE OPENED THE
DOOR FOR J.J. THOMPSON
electron
e
-1
0
9.11x10-28
proton
+
p
+1
1
1.67x10-24
neutron
n0
0
1
1.67x10-24
THE ATOMIC NUCLEUS
 WHEN
SUBATOMIC PARTICLES WERE
DISCOVERED, SCIENTISTS WONDERED
HOW THESE PARTICLES WERE PUT
TOGETHER IN AN ATOM.
 THIS WAS A DIFFICULT QUESTION TO
ANSWER, GIVEN HOW TINY ATOMS ARE.
 MOST SCIENTISTS THOUGHT IT LIKELY
THAT THE ELECTRONS WERE EVENLY
DISTRIBUTED THROUGHOUT AN ATOM
FILLED UNIFORMLY WITH POSITIVELY
CHARGED MATERIAL.
• BASED ON THE RESULTS,
RUTHERFORD SUGGESTED A NEW
THEORY OF THE ATOM.
• HE PROPOSED THAT THE ATOM IS
MOSTLY EMPTY SPACE
–THIS EXPLAINED THE LACK OF
DEFLECTION OF SOME OF THE
PARTICLES
• HE CONCLUDED THAT ALL THE
POSITIVE CHARGE AND ALMOST
ALL THE MASS IS CONCENTRATED
IN A SMALL CORE
–HE CALLED THIS REGION THE
NUCLEUS
• IT’S THE NUMBERS OF PROTONS IN
AN ATOM THAT MAKES ATOMS
DIFFERENT.
– AN OXYGEN ATOM HAS 8 PROTONS
IN ITS NUCLEUS
– A HYDROGEN ATOM HAS 1 PROTON
IN ITS NUCLEUS
• THE ATOMIC NUMBER OF AN
ELEMENT IS THE NUMBER OF
PROTONS IN THE NUCLEUS OF AN
ATOM OF THAT ELEMENT
– SINCE OXYGEN’S NUCLEUS HAS
PROTONS ITS ATOMIC # IS 8
• THE PROTONS IDENTIFY THE TYPE
OF ATOM, THE ONLY ATOM WITH
8 PROTONS IS OXYGEN
8
ATOMS OF THE 1ST TEN ATOMS
Lithium
H
He
Li
1
2
3
1
2
3
0
2
4
1
4
7
1
2
3
Beryllium
Be
4
4
5
9
4
Boron
B
C
N
O
F
Ne
5
6
7
8
9
10
5
6
7
8
9
10
6
6
7
8
10
10
11
12
14
16
19
20
5
6
7
8
9
10
Hydrogen
Helium
Carbon
Nitrogen
Oxygen
Fluorine
Neon
• FOR EACH ELEMENT LISTED, THE
NUMBER OF PROTONS EQUALS THE
NUMBER OF ELECTRONS
– REMEMBER ATOMS ARE
ELECTRICALLY NEUTRAL
• IN AN ATOM, THE NUMBER OF
ELECTRONS MUST EQUAL THE
NUMBER OF PROTONS
• A HYDROGEN ATOM HAS 1 ELECTRN
• AN OXYGEN ATOM HAS 8 ELECTRNS
• THE MASS OF AN ATOM IS CONCEN.
IN ITS NUCLEUS AND DEPENDS ON
THE NUMER OF PROTONS AND
NEUTRONS
• THE TOTAL NUMBER OF PROTONS
AND NEUTRONS IN AN ATOM IS
CALLED THE MASS NUMBER
– A CARBON ATOM, WHICH HAS 6
PROTONS AND 6 NEUTRONS, HAS A
MASS NUMBER OF 12
• IF YOU KNOW THE ATOMIC NUMBER
AND MASS NUMBER OF AN ATOM OF
ANY ELEMENT, YOU CAN
DETERMINE THE ATOM’S
COMPOSITION
• THE COMPOSITION OF ANY ATOM
CAN BE REPRESENTED IN
SHORTHAND NOTATION:
CHEMICAL COMPOSITION
SHORTHAND
# OF PROTONS
+
# OF NEUTRONS
MASS
NUMBER
35
ATOMIC
NUMBER
17
Cl
NUMBER OF
PROTONS
ISOTOPES
• EVERY CHLORINE ATOM HAS 17
PROTONS, WITHOUT EXCEPTION,
– HOWEVER, NOT EVERY CHLORINE
ATOM HAS
18
NEUTRONS.
– ATOMS WITH THE SAME NUMBER OF
PROTONS BUT CONTAIN DIFFERENT
NUMBERS OF NEUTRONS ARE
CALLED ISOTOPES.
• BECAUSE ISOTOPES OF AN
ELEMENT HAVE DIFFERENT
NUMBERS OF NEUTRONS THEY HAVE
DIFFERENT MASS NUMBERS.
• ISOTOPES ARE CHEMICALLY ALIKE
BECAUSE THEY HAVE IDENTICAL
NUMBERS OF PROTONS AND
ELECTRONS
– IT’S THE ELECTRONS AND PROTONS
THAT ARE RESPONSIBLE FOR
CHEMICAL BEHAVIOR
BERYLLIUM
ISOTOPES
proton
neutron
electron
EXAMPLE OF AN
ISOTOPE
ATOMIC MASS
Cl
Cl
35
37
17
17
18
NEUTRONS
20 NEUTRONS
ATOMIC NUMBER
 AN
IONS
ELEMENT’S ATOMS ARE NOT
ALWAYS NEUTRAL IN CHARGE.

WHEN AN ATOM LOSES OR GAINS ONE OR
MORE OF ITS ELECTRONS IT BECOMES ION.
 AN
ION THAT HAS MORE ELECTRONS
THAN PROTONS HAS A NEGATIVE
ELECTRICAL CHARGE
 AN ION THAT HAS FEWER ELECTRONS
THAN PROTONS HAS A POSITIVE
ELECTRICAL CHARGE
NOTE: IT’S THE PROTONS THAT DEFINE
THE TYPE OF ATOM IT IS, BUT THE
ELECTRONS DEFINE THE ATOM’S
CHARGE.
SOME ATOMS
GAIN ELECTRONS
ATOM’S IONIC CHARGE =
# PROTONS - # ELECTRONS
-
-
-
-
-
-
-
O
-
-
-
-
-
-
-
O-2
-
ATOMS, IONS, AND ISOTOPES
ATOMS
NEUTRAL AND ARE DEFINED BY THE #
OF PROTONS IN THEIR NUCLEUS
3 p+ = Li ATOM, ETC.
IONS
HAVE AN ELECTRICAL CHARGE
DETERMINED BY
PROTONS - # ELECTRONS
N-2 = 7 p+ - 9 e- ; ETC.
ISOTOPES
TWO ATOMS WITH THE SAME # OF
PROTONS, BUT DIFFERENT #’S OF
NEUTRONS OR MASSES
CALCIUM-40 & CALCIUM-44
#